Hey, I'm about to trade these items. How many diamonds would make this a good trade for me, in your opinion?

I would say at least 500, based on the value of the helds. Maybe up to 900-1000 depending on how you value the snowy owl.

It depends on (1) How desperate you need gold/diamonds (2) How much the other people actually want the items.

My rough estimates:

Non-held
Qitari Tail - Since may be re-released during spring festival, No more than 200k gold.
Nordic Outfit - Probably around 150k gold at the moment due to seeing a few on the broker recently. It'll probably go up in value as time passes and less are available.
Costume thing - I'm not sure, probably 200-250k gold going off the broker.
- - -
~600k => 200-250dia.

Held
(Can't remember a few names off the top of my head -- these are tougher to price)

Ryori - While the water one isn't on the broker atm, others are for 500-750k. Probably around 700k (Due to its age and availability)
Frosty - 300 dia (Foxes are popular + exclusive)
Red Fox thing - 400 dia (Foxes are popular + good colors)
Light blue sitting dragon - 400 dia (These were a pain to get xD)
guardian - 250 dia (Not great colors)
aquatic blue thing - 250 dia (Value of being old)
aquatic purple thing - 250 dia (Value of being old)
white owl thing - 300 dia (Exclusive)
- - -
~ 700k + 2250 dia => 2,350 dia


All together => 2,650 dia (Roughly 6.8 mil gold)
But I'm giving more towards the upper end for price values for the most part. For the general population though, people who like but don't love the items, probably worth 2k dia instead. (Roughly 5.2 mil gold)

But in the end, it all comes down to how much the buyers value things. Like for me, I'm not fond of frosty, so personally I'd pay at most 100 dia, versus I find the red fox really adorable and would pay closer to 400 dia if I didn't have one already. :)

Thank you! This helps alot!